1. Field of the Invention
This invention relates to methods and apparatus for feeding an optical signal into an optical fiber through the buffer thereof and for withdrawing an optical signal from an optical fiber through the buffer thereof.
2. Introduction to the Invention
The most widely used optical fibers comprise a glass core, a glass cladding which surrounds the core and a buffer which surrounds the cladding and is composed of relatively soft polymeric material. Where reference is made in this specificatioon to an "optical fiber" or an "optical fiber as defined", it is to be understood that the fiber referred to is one comprising a glass core, a glass cladding and a buffer having a refractive index greater than the cladding. The buffer protects the cladding and the core and allows the fiber to be bent into (and maintained in) a bend of substantially smaller bend radius than would otherwise be possible. The buffer may be surrounded by a jacket. Optical signals are conventionally fed into optical fibers by directing them axially at the exposed fiber end. It has also been proposed to feed an optical signal into a fiber through the side of the cladding, after removal of any buffer in the path of the signal.
It is often desirable to bring two optical fibers into precise axial alignment, so that a signal can be passed from one fiber to the other with minimum loss. Often the aligned fibers will then be joined together. The joint can be a permanent joint (usually called a "splice") made by joining the fibers directly to each other, either by fusing the fibers together or by adhering them to each other with an index-matching adhesive (i.e. an adhesive whose refractive index is chosen so that signal loss is minimized). Alternatively the joint can be a disconnectable one made by mounting the fiber ends in two mating parts of a connector and then joining the two mating parts together in such a way as to ensure alignment of the fibers.
The known methods for aligning optical fibers as defined suffer from serious disadvantages. Most of the known methods align the exteriors of the two fibers, not their cores (whose alignment is of course the dominant factor in determining the signal loss at the joint). This type of process is very much dependent on operator skills and in any case is likely to lead to poor results because the core is often not precisely in the center of the fiber. It has also been proposed to join the ends of two optical fibers by bringing the ends into approximately aligned and abutting relationship, feeding an optical signal into the remote end of one fiber, detecting the optical signal at the remote end of the other fiber, and moving the to-be-joined ends so as to maximise the detected signal, thus precisely aligning the cores in those ends. This method is capable of yielding excellent results, but is wholly impractical because of the need to establish communication links between the alignment point and the remote ends of the fibers. Borsuk, U.S. Pat. No. 4,215,937 proposes a method in which an optical fiber is centered in one part of a connector, by an optical method which makes use of an optical signal which is introduced into the fiber through the cladding, without passing through a buffer.